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Options for building rice field fertility

At the recent Missouri Rice Farm field day outside Glennonville, Mo., Gene Stevens tackled two soil-related topics: chlorine fertility in rice and the economics of building up phosphorus and potassium in a low testing field.

“You may remember from botany class that there are 16 essential elements required for plant growth,” said the Missouri Extension soil fertility specialist based at the Delta Center in Portageville, Mo. “Nitrogen, phosphorus and potassium are the macro nutrients. There are also micronutrients required… One of the last micronutrients found — actually discovered in the last century — was chlorine. The reason it was discovered so late is plants use such a small amount of it.

“We’ve never really been concerned about it from a fertilizer point of view because whenever we apply potassium as potash (potassium chloride), chlorine is being applied at the same time.”

Carbon is fixed in the plant through carbon dioxide in the leaves while hydrogen and oxygen are split from water. Chlorine is integral in allowing the plant to perform those functions.

Current research in the Bootheel is looking at chlorine added to Roundup in soybeans. “Chlorine apparently has an anti-fungal property to it in plants. Two years ago, we began experimenting with foliar chlorine because of the ASR scare. That work has since expanded to other diseases likes frogeye leafspot.”

Stevens is working with research colleague, David Dunn, on testing chlorine in rice.

“Dunn applied ammonium chloride preplant and also balanced fertilizer treatments with urea so we would know if rice yield response was from chlorine or nitrogen in the ammonium. We found that ammonium chloride shortened the rice internodes compared to no-chlorine checks.

“That can be good or bad. If you apply ammonium chloride to a rice variety that’s prone to lodge, it may help.”

The 2005 data was gathered from different rates of chlorine — up to 50 pounds in 10-pound increments. The researchers found a trend, “about a 15-bushel increase” from applying 20 and 30 pounds of chlorine.

“That’s very surprising to me. We really did not expect any response to chlorine. And one year’s data isn’t enough to make definite conclusions. But we’re studying it again this year and probably will continue.

“This field,” said Stevens pointing behind him, “didn’t have potassium chloride (potash) applied in 2005. So Dunn’s looking at a relatively untouched field. For some reason, chlorine doesn’t seem to build up in soils as we might expect. If you’re putting potassium chloride out year after year, you’d think chlorine would be there. But since it’s a negative ion, it tends to leach. At least that’s what we think is happening — there’s a lot we don’t know about chlorine because it hasn’t been studied much.”

Fertilizer build-up

When sending a soil sample to a University of Missouri soil lab, producers often ask for fertilizer buildup options.

“For example, if you have a field testing low for phosphorus and potassium, you can specify how many years you want to allow for building a field to optimum fertility levels. If you’re in a tight budget, you probably don’t want to try and correct a fertility problem in a single year. Normally, because of cash flow limitations, it takes a while to build up a low-testing soil.”

But in some cases a producer may want to go with a shorter time span. For example, “maybe you’ve had a field grid soil-sampled and want to shorten the buildup time to save the added expense of variable rate fertilizer application for several years.”

Of course, besides the buildup component there’s also the crop-removal component. A rice/soybean rotation removes about the same amount of phosphorus annually.

In an experiment being conducted in plots behind him, Stevens said, “140 bushels of rice per acre removed about 41 pounds of phosphorus. For 45-bushel soybeans, 39 pounds was removed.

“However, with potassium, soybeans remove much more than rice. In a good program, you need to take that into consideration.”

To bring a field up to optimum fertility, Stevens provided a chart for acreage in continuous rice. The fertilizer buildup data was for one-year, four-year and eight-year periods.

“Add the dollars across years, and I’m reminded of the slogan in an oil filter commercial: ‘You can pay me now or pay me later.’ The same total amount of fertilizer is applied in each buildup program over eight years, just with more up front in the one-year and four-year programs compared to the eight-year buildup program.

“For the one-year buildup, fertilizer costs averaged $72 per acre for 2004 and 2005 in a rice/soybean rotation. The four-year test cost $57 per acre and the eight-year cost $49.”

The interesting thing is the one-year buildup cost is already committed. From now on in the experiment, “we will only apply crop-removal phosphorus and potassium to those plots. Even though your startup costs are high in a short buildup program, from then on the costs drop dramatically.

“Assuming fertilizer prices would be constant in our longest buildup treatment, the cost will be $49 for all eight years.”

Yields and net returns are much higher in the one-year buildup than the other two programs. That’s because soil test phosphorus and potassium levels were initially low in the fields.

For the economics, subtract gross value of rice/soybeans minus fertilizer.

“In 2004/2005, we got $23 per acre more in the one-year buildup even though we had a big fertilizer expense up front. The other programs provided $19 and $17.”

In a situation where a farmer owns his land with low soil tests, “it’s probably most economical to build the fertility up as quickly as he can. If he’s renting and doesn’t have a really long lease, he probably will want to try a longer buildup.”

Stevens is conducting the same experiment with fescue.

“We’re getting really big yield responses. But the value of fescue is so low, the economics of fertilizer buildup are much different. That means the value of the crop has a big influence on the economics of optimum buildup time.”

Also, in almost all situations, producers don’t sample soils yearly. Most sample every three years. One problem with fertility programs is it takes three reference points to have confidence in a trend.

“If you go nine years before verifying a trend, it’s a long time to wait to change course in a fertility program. So we’re looking closely at our basic equations to make sure recommended fertility programs are what they should be to build soils to their optimum levels.”